The present invention relates to the field of microgenerators of electrical energy. In particular, the invention regards a microgenerator designed for a wide range of different applications, and having a high degree of efficiency and in particular a high power density. A preferred example of application regards the use of the microgenerator according to the invention for the supply of portable electronic apparatuses. In this field, it is of primary importance to provide a power density (power/mass ratio [power]) that is as high as possible. The solutions up to now proposed in the art are not completely satisfactory from this standpoint.
Miniaturized solar cells and lithium microbatteries have been proposed as integrated power sources for MEMS applications. The estimated power density of said embodiments is in the region of 1 MW/m3. Solid-state lithium microbatteries of a rechargeable type have a power density in the region of 0.4 MW/m3.
Electric micromotors have a power density in the region of 1.7 MW/m3. The magnetic micromotors manufactured by Ahn and Allen (Proc. Microelectrical Mechanical Systems, IEEE Robotics and Automation Society, pp 1-6, 1993) have a power density of 200 MW/m3.
Chemical reactors are not designed for the generation of energy, but an exothermal reaction of partial oxidation can generate a non-negligible amount of energy. The T microreactor illustrated by Jensen et al. (Microreaction Technology, Proceedings of the First International Conference on Microreaction Technology, 1997), used for studying the catalytic oxidation of ammoniac had a power density in the region of 20 MW/m3.
There have also been used conventional devices for generating energy on a small scale. For example, metal-channel flow reactors, which use controlled H2—O2 reactions, have been used as heat exchangers and evaporators. In particular, with a Pt-catalysed H2—O2 reaction there has been reported a power density of 150 MW/m3 (Hagendorf; Process Miniaturization: 2nd International Conference on Microreaction Technology, Topical conference reprints, 1998).
The heart of a polymeric-electrolytic-membrane fuel cell (PEMFC) is the membrane-electrode assembly (MEA) made up of catalysed anode and cathode electrodes joined or applied on a side of a membrane made of solid polymeric electrolyte. In a direct methanol fuel cell (DMFC), the methanol can be oxidized directly into carbon dioxide and water on the catalytically active anode without any equipment for pre-treatment of the fuel. The main advantage of the DMFC is the elimination of the fuel processor: this gives rise to a simpler operation and to an operation presenting greater reliability, smaller volume and lower operating costs. Surampudi et al. (J. Power Sources, 47 vol. 377, 1994) has produced a high-performance DMFC at a temperature of 88° C. with a concentration of 2 M of methanol. Shukla et al. (J. Power Sources, 55, vol. 88, 1995) and Arico et al. (J. Electroch. Soc., 143, vol. 3950, 1996) have achieved a power density of around 200 MW/cm3 at 900° C.
A modern turbine engine for aircraft, with a fuel flow rate of 4 kg/s produces approximately 150 MW of power in a typical combustion chamber of 0.1 m3. This corresponds to a power density of 1500 MW/m3.
The silicon microcombustor, as proposed for the first time at the Massachusetts Institute of Technology, which is characterized by a power density of 2300 MW/m3, is obtained using micro-electronics technologies with a complex process, which involves 7 aligned-wafer connections, 20 lithography steps, and the deposition of 9 thin-film layers.
The main engine of the Space Shuttle, with a flow rate of hydrogen fuel of 75 kg/s, produces approximately 9000 MW of power. Using a combustion-chamber volume of 0.13 m3, the resultant power density is 70000 MW/m3.
The purpose of the present invention is to provide a microgenerator of electrical energy which is characterized by a high power density, for example in a band which ranges from 1000 to 10000 MW/m3.